Weeds and tall pasture grass should be no problem for a finish mower but the more important question is "what else are you going to run over out in the pasture?" Hitting old stumps, bottles, rocks etc would damage a finish mower but would only cause a nasty noise when hit by a "brush hog". If the pasture, when cut, really looks like a lawn and if you are reasonably sure there are no hidden surprises, then the finish mower could do the job. If you have doubts, see if you can rent a brush hog at least the first time you mow.